This invention relates to methods and devices for use in clinical medicine particularly to provide the integration of the measurement of oxygen saturation of arterial blood and the associated minute ventilation from which the oxygen saturation is derived to enhance the clinical utility of the oxygen saturation measurement.
The brief measurement of oxygen saturation by pulse oximetry in the assessment of shortness of breath is a standard medical practice. This is commonly known as xe2x80x9cspot (oximetry,xe2x80x9d as is differentiated from continuous oximetry monitoring in that the oxygen saturation measurement is made for a brief period of time (such as 30 seconds to 5 minutes) and then generally reported as a single value (such as the average oxygen saturation or lowest oxygen saturation). There has been a trend toward smaller, more compact oximeters, which do not record or print out the saturation values but rather provide a digital reading of the number (such as xe2x80x9c96%xe2x80x9d). Unfortunately, this trend toward using a single spot xe2x80x9cnumber,xe2x80x9d while more simple and convenient, does not take into account the dynamic interrelated mechanisms operative to generate the oximetry number.
Spot oximetry provides important information, is useful as a screening test, and is often employed as an additional xe2x80x9croutine 5th vital signxe2x80x9d along with temperature, pulse, respiration, and blood pressure. Unfortunately, spot oximetry measurement has serious limitations which are often not considered by physicians and respiratory therapist is ordering them. To understand the clinical significance of these limitations and their potential to adversely affect patient care, it is important to understand how spot oximetry is used in clinical medicine on the hospital wards.
Spot oximetry provides important information, is useful as a screening test, and is often employed as an additional xe2x80x9croutine 5th vital signxe2x80x9d along with temperature, pulse, respiration, and blood pressure. Unfortunately spot oximetry measurement has serious limitations which are often not consider by the physicians and respiratory therapist ordering them. To understand the clinical significance of these limitations and their potential to adversely affect patient care, it is important to understand how spot oximetry is used in clinical medicine on the hospital wards.
Commonly, indeed thousands of times each day, a respiratory therapist, intern, resident or other physician is called to see a hospitalized patient for the evaluation shortness of breath. Often the first diagnostic test ordered is a spot oximetry test. Many medical personnel think of this measurement as pivotal. The thinking goes like this, xe2x80x9cif the spot oximetry measurement is within the normal range then the shortness of breath is much less likely to reflect a problem which is life threatening.xe2x80x9d This is a common misconception and leads to the general and widespread use of spot oximetry on hospital wards as a test to assess the immediate clinical significance of shortness of breath. Unfortunately, as will be discussed, spot oximetry is a poor indicator of the immediate clinical significance of shortness of breath and a normal spot oximetry value commonly provides the health care worker with a false sense of security which can lead to a poor patient outcome due to delayed diagnosis.
The present inventor has long attempted to instruct interns and medical residents of the hazards of using spot oximetry in this way. But teaching alone cannot correct this ubiquitous problem. Even within the hospital of the present inventor, each year the stability of the respiratory state of many patients is misconstrued by interns and respiratory therapists by the false sense of security provided by a normal spot oximetry value, and timely patient treatment is delayed. Given the frequency of this problem within a hospital wherein active teaching of the hazards of spot oximetry is provided, it is likely that nationwide, thousands of patients have delayed intervention every year due to the improper interpretation of the significance of a normal spot oximetry measurement.
When considering the management of critically ill patients, the timing of intervention is of paramount importance. The value of any diagnostic test is in part determined by the ability of the test to predict adverse physiologic events early in the course of the disease process wherein intervention is more likely to be effective. This is particularly true of the diseases associated with infections such as toxic shock, sepsis, and pneumonia or with thromboembolic disease. Regrettably, with respect to early identification of impending respiratory failure or severe respiratory dysfunction, spot oximetry is very poor.
The lack of utility of spot oximetry as an early indicator of impending respiratory failure has as it""s physiologic basis the position the overall significance arterial oxygen saturation has to human cellular respiration and survival. The arterial saturation, as determined by the spot oximetry test, is one of the most important parameters defining the sufficiency of lung function and oxygen delivery to the tissues. This is one of the reasons that physicians give a normal reading such significance, but it is also the reason that the human body protects oxygen delivery by keeping the value of arterial oxygen saturation close to the normal value till death is very near. In humans, a primary protective mechanism is immediately operative to keep the arterial oxygen saturation value high until late in the course of critical illness such that the patient will increase the volume of air breathed per minute which will xe2x80x9cblow off carbondioxidexe2x80x9d and which will raise the oxygen saturation. (With a normal respiratory quotient the partial pressure of oxygen increases by 1.2 mm for each 1 mm the CO2 falls, while the associated rise in oxygen saturation in response to a fall in CO2 is dependent on the position of the oxygen saturation on the oxyhemoglobin dissociation curve). This often occurs early in the course of the illness (such as sepsis) before any fall in oxygen saturation has occurred and can be seen as a xe2x80x9cpreventivexe2x80x9d survival mechanism protecting the patient against a fall in oxygen saturation before it occurs. The fall in CO2 caused by the increased minute ventilation also increases the affinity of hemoglobin for oxygen, this means that at any given level of partial pressure of oxygen, the arterial oxygen saturation as measured by a spot oximetry device is higher. In other words, despite a decline in the lung""s ability to keep the dissolved arterial oxygen in the normal range the spot oximetry reading can still be entirely normal due to this compensatory mechanism.
A final confounding factor is anxiety. Anxiety causes an increase in minute ventilation, which raises the spot oximetry value due to the mechanisms noted supra. Unfortunately, in some patients, the increased minute ventilation can be associated to a greater extent with larger volume breaths than with a marked increase in respiratory rate. For this reason, increased minute ventilation may be subtle and not recognized during clinical assessment. The classic case is the anxious young woman with a vaginal discharge. The spot oximetry value may be normal even as she is dying of toxic shock. Her body is protecting the oxygen delivery to the tissues and the anxiety is driving the CO2 lower further increasing the spot oximetry value. But the intern, or emergency room physician, may misconstrue the anxiety and the increased minute ventilation and fail to recognize the impact it has on the spot oximetry value. The physician seeing a perfect spot oximetry value of 97% for example may feel reassured that the patient is not in danger of respiratory failure during the night. However, with toxic shock there is no time to lose. When the compensating mechanisms are exhausted, such patients deteriorate rapidly, often over 6-12 hours, too rapidly for the physician seeing the patient the next day to intervene.
In addition to the problems noted above there is another mechanism by which the common application of spot oximetry is adversely affecting timely medical intervention. From a more scientifically enlightened perspective the spot oximetry can be seen as a simple test which can actually only provide information about the arterial oxygen saturation and pulse. When applied in this limited manner, the test is both sensitive and specific for a fall in arterial oxygen saturation, which is indeed an important abnormality to identify. However, this is not the way the test is used clinically. The test is commonly used as an initial screen, as an initial substitute for an arterial blood gas in the evaluation of shortness of breath. Indeed a very common mistake is to start with a spot oximetry test and if that is normal to dispense with the blood gas test, which is more painful, time consuming and costly. This is due to the fact that many health care workers do not recognize that spot oximetry is an insensitive test when applied to determine the presence of even severe respiratory or metabolic dysfunction. When used in this way the test is being applied in a much larger manner than the role for which it is suited i.e. it is being used, as a preliminary substitute for an arterial blood gas to initially assess the respiratory status of the patient generally. Spot oximetry cannot provide that function. In fact when used in that manner the spot oximetry test has the potential to cause significant harm as a function of delayed diagnosis, by providing a false impression that an arterial blood gas is not necessary. A patient with metabolic acidosis, for instance, due to late sepsis, will often have superimposed respiratory alkalosis and a normal spot oximetry study. The spot oximetry study, when used to determine the need for an arterial blood gas will, in this case, commonly be normal until very late (too late). If a spot oximetry test is used as an initial screen, the blood gas may not be obtained and the diagnosis of this volatile and potentially fatal critical condition may be seriously delayed.
Education is not the solution; the hazards of the spot oximetry test are too abstract for many physicians and therapists. Many do not fully understand the complex interactions of the dynamical physiologic systems defining an evolving critical illness. Also many healthcare workers have not had sufficient training in the physiology of critical illness to fully grasp the limited relevance of isolated xe2x80x9cspotxe2x80x9d measurements of instantaneous parameters at any given point along the time continuum. In addition the dynamical interactions of multiple compensatory mechanisms are pivotal and in aggregate these issues are simply too complex for many non-physiologist clinicians. Yet every physician and respiratory therapist in the hospital is expected to understand a spot oximetry test as if it were as simple as a temperature measurement.
The present inventor became frustrated with the difficulty associated with attempts to mitigate this problem through education of interns and residents, and in discussing the problem with other pulmonary specialists, he found the problem to be ubiquitous. To solve this problem the present inventor designed a simple portable medical device which provides enhanced information relating to magnitude of operative compensatory mechanisms relating to the gas exchange status of the patient and therefore is superior to the measurement of spot oximetry for the assessment of shortness of breath on hospital wards. The device further provides a mechanism to identify those patients wherein the spot measurement is likely to spuriously suggest normality. And further it is a portable, hand-held device, which can rival spot oximetry in its simplicity. For these reasons, it provides an improved initial screening test for shortness of breath in that, like an arterial blood gas, it is a more sensitive indicator of the presence of evolving respiratory instability than the spot oximetry but, like spot oximetry, it is both simple and inexpensive to apply. Further the testing is non-invasive and does not cause pain. Perhaps most importantly, the device generates xe2x80x9ca simple numberxe2x80x9d, xe2x80x9ca parameterxe2x80x9d which combines the value of the oximetry reading and the magnitude of compensatory or confounding mechanisms. This can aid in simplifying the assessment process and reduce the potential for errors. Since the xe2x80x9cnumberxe2x80x9d generated includes automatic adjustments for compensation, the physician or therapist is provided with a more accurate picture of the status of the respiratory system. The pragmatics, the xe2x80x9creal worldxe2x80x9d of this issue cannot be overstated. It is critical that any device competing with spot oximetry for a place as an initial screening test of shortness of breath be very simple, inexpensive, noninvasive, portable, compact, and it must generate a simple xe2x80x9cnumberxe2x80x9d or index which the physician or therapist can use along with clinical assessment to decide whether the pain, and expense of an arterial blood gas measurement is indicated. It is the purpose of the present invention to provide all of these features as well as other features as will be evident from the discussion which follows.
The present invention uses a comparison of the minute ventilation with the oximetry measurement to determine indexed oxygen saturation. As discussed supra, in health, the oxygen saturation is increased by a rise in minute ventilation as a function of the effect such rise has to increase alveolar oxygen partial pressure and to reduce carbondioxide levels. Since all patients have deadspace ventilation, and since the dead space ventilation is variable from patient to patient and even within the same patient with different tidal breaths, it is not possible to reliably determine the true alveolar pCO2 (or alveolar O2) by measuring the minute ventilation alone. However precise determination of these numbers is not necessary. Indeed the spot oximetry value itself is not a precise measurement of the arterial oxygen saturation of hemoglobin but is rather an approximation. It can therefore be seen that both spot oximetry and the ventilation indexed oximetry test of the instant invention are general parameters intended to be applied along with clinical assessment to determine the need for the more precise testing associated with arterial blood gas measurement.
Generally one presently preferred embodiment of the present invention is a ventilation indexing oximeter including a ventilation measurement device for generating a first result indicative of the volume of gas ventilated by a patient per unit of time, an oximeter for generating a second result indicative of the oxygen saturation of human blood corresponding to the first result, and a microprocessor programmed for comparing said first result with said second result to generate a third result indicative of both said first result and said second result.
The ventilation-measuring device preferably comprises a flow sensor capable of determining the minute ventilation of a patient. The first result, which can be the average minute ventilation, can be determined for a first time interval. The second result, which can be the average oxygen saturation of arterial blood as determined by pulse oximetry, can be determined for a second time interval which can commence after or near the end of the first time interval. The first time interval is sufficient for said patient""s respiratory status to become stabilized. The second time interval can be shorter than said first time interval. The third result can be a calculated index of said first result and said second result.
In one preferred embodiment, the present invention provides a microprocessor system generating a mathematical comparison of concomitantly or near concomitantly measured oxygen saturation and timed ventilation gas measurement. In the preferred embodiment, a mathematical comparison is provided of the average arterial pulse oximetry measurement and the associated measured average minute ventilation (timed ventilated volume) of all ventilated gasses within a given measurement interval. One preferred embodiment provides a microprocessor to calculate the index as the measured minute ventilation divided by the predicted resting normal minute ventilation for the patient""s age, body surface area, and sex (as is well known in the art). The Oximetry measurement is indexed to adjust for the potential effect of increased minute ventilation to increase the saturation.
It is the purpose of the present invention to provide a microprocessor system, which compares the measured arterial saturation to the timed ventilation from which it was derived.
It is further the purpose of the present invention to provide an index, which improves the clinical utility of the measured pulse oximetry value.
It is further the purpose of the present invention to provide a portable, handheld ventilation indexing oximeter to measure and display the arterial oxygen saturation and to indicate the corresponding minute ventilation from which it was derived provide an improved, immediate clinical assessment of shortness of breath at the bedside.
It is further the purpose of the present invention to provide a continues moving mathematical comparison of the arterial oxygen saturation and the corresponding ventilated gas measurement from which it was derived so that the magnitude of change in ventilated gas measurement required to induce a given magnitude of change in arterial saturation can be optimally assessed.